It is known to anneal steels containing up to 100 ppm boron in the dissolved form at an elevated temperature in the range around 1000.degree. C. In this treatment, it has been observed repeatedly that the products are undesirably impoverished in boron, at least in the surface area. Thus, for example, T. Inoue and Y. Ochida described, in Lecture No. S. 1351, presented at the 102nd ISIJ Meeting 1981, that they had found, a boron impoverishment to a depth of up to 0.4 mm from the surface, after annealing of low-alloy steel containing 0.2% carbon and 30 ppm boron at 900.degree. C. for 2 hours under flowing argon. The same result was obtained when annealing was performed in air instead or argon. They therefore recommended that boron-containing steels should be annealed in a vacuum of 13.3 mPa in the presence of a zirconium getter.
P. E. Busby, M. E. Warga and C. Wells reported in the Journal of Metals, November 1953, pp. 1463-8, that they had observed a simultaneous evaporation of carbon and boron from a low-alloy steel having 0.43% carbon and 38 ppm boron during wet hydrogen in annealings in the austenite range--i.e., above about 900.degree. C.
In the Journal of Metals, February 1954, pp. 185-190 M. E. Nicholson reported that he had detected a fairly heavy enrichment of boron in the core of the samples, ascribing this to the formation of boron compounds. He recommended that, when adding boron, more particularly the oxygen content should be taken into account, since a portion thereof would combine with boron to form boron oxide, thus reducing the soluble proportion of boron having a hardenability-enhancing effect.
Frequently boron-containing steels exhibit very different hardenability. Since no systematic investigation has been carried out in this respect, the causal connection has hitherto remained unclarified.
Boriding is described in the specialist book "Boriding" by A. Graf von Matuschka (1977), published by Carl Hanser. Boriding processes are also known from German OS 2 126 379 and British Patent Specification 1,435,045.